1. Field of the Invention
The present invention relates to a heat-sensitive recording paper and, more particularly, to a heat-sensitive recording paper capable of forming a dye image having improved preservability under high temperature and high humidity conditions.
2. Description of the Prior Art
In heat-sensitive recording papers images are formed utilizing physical or chemical changes in the materials induced by heat energy, and a large number of processes using heat-sensitive recording papers have been investigated.
One type of heat-sensitive recording paper which relies upon a heat induced physical change is a so-called wax type heat-sensitive recording paper is well known and has been used for recording electrocardiograms and the like. On the other hand, as a material utilizing a chemical change due to heat to form images, a number of materials based on various different color-forming mechanisms have been proposed. Of these, a binary coloration system is most typical.
A binary coloration system is prepared by dispersing two heat-reactive compounds as fine particles in a binder or the like such that the binder separates them from each other, and coating the resultant mixture on a base paper. The thus prepared paper records images by heating it to melt one or both of the compounds such that they came into contact with each other, thus causing a color-forming reaction. These two heat-reactive compounds are generally referred to as electron donating compounds and electron accepting compounds, respectively. An extremely large number of combinations of them are known, and they are roughly classified as systems forming images of a metal compound and systems forming dye images.
Representative examples of systems forming images of a metal compound are those in which the electron donating compounds are organic reducing agents such as phenols, chelating agents, sulfur compounds, or amino compounds, and the electron accepting compounds are organic metal salts. These two produce a metal, metal complex compound, metal sulfide, or the like through the reaction therebetween upon being heated, thus providing a colored image. Specifically, there are a combination of thiourea and heavy metal salt (U.S. Pat. No. 2,740,895), a combination of gallic acid or the like and metal salt of stearic acid (U.S. Pat. Nos. 2,663,654, 2,663,655, 2,663,656, and 2,663,657), a combination of hydroquinone and silver behenate (U.S. Pat. No. 3,031,329), a combination of hexamethylenetetramine and tin compound (U.S. Pat. No. 2,813,043), and the like.
On the other hand, examples of systems forming dye images are those in which the electron donating compound is an electron donating colorless dye, and the electron accepting compound is an acidic material such as phenol or the like (Japanese Pat. No. 4,160/68 and U.S. Pat. Nos. 3,451,338; 3,539,375; 3,674,535 and 3,666,525).
These binary coloration systems of heat-sensitive recording papers have many advantages as recording papers. For example: (1) they are based on primary coloration and do not require development processing; (2) the paper quality is similar to that of ordinary paper; and (3) they are handled with ease. In particular, those materials wherein a colorless dye is used as the electron donating compound have the additional advantages that (4) they provide higher color density and (5) they enable one to easily obtain heat-sensitive recording papers forming different colors, thus being more valuable. Therefore, they are in most use as heat-sensitive recording papers.
Above all, those papers in which a bisphenol derivative represented by the following general formula: ##STR1## wherein R.sub.1 and R.sub.2 each represents a hydrogen atom or an alkyl group containing 1 to 12 carbon atoms (e.g., R.sub.1 may be a hydrogen atom, a methyl group or an ethyl group while R.sub.2 is a methyl, an ethyl, a propyl, an n-butyl, an n-octyl, an n-decyl, an iso-butyl, or an iso-heptyl group) or, when taken together, R.sub.1 and R.sub.2 form a saturated carbocyclic ring (i.e., cyclohexyl), is used as an electron accepting compound exhibit high coloration sensitivity and provide high coloration density. However, they have several defects. One defect is that they form increased background fog when stored under high temperature and high humidity conditions. Conventional binary coloration system of heat-sensitive recording papers from an increased background fog during storage under high humidity even when the temperature is lower than the temperature at which the coloration reaction takes place. This is not favorable in the point of storage stability. This tendency is particularly serious when paper is used as a support as compared with the case when a plastic film or metal is used as a support.